My invention relates to magnetic tape recording and reproducing systems, particularly to those having a rotary electromagnetic head assembly, suitable for the recording and reproduction of video signals on magnetic tape packaged in cassette form. However, I do not wish may invention to be limited to this particular application because the fundamental concepts of my invention are applicable to digital audio tape cassette apparatus and other magnetic tape devices of the kind having a rotary head assembly.
In a typical rotary head video tape cassette apparatus, as is well known, a pair of electromagnetic data transducers or heads are mounted to a rotary drum in diametrically opposite positions thereof to make up a rotary head assembly. The heads are electrically connected to a recording circuit and a reproducing circuit via a "rotary" transformer. As heretofore constructed, the rotary transformer had a "rotary" winding mounted to the rotary drum and electrically connected to each head, and a "fixed" winding mounted to a stationary part of the apparatus and electromagnetically coupled to the rotary winding.
Conventionally, the single fixed winding of the transformer was used both for recording and reproduction; that is, the fixed winding had its opposite extremities connected respectively to the recording circuit and to the reproducing circuit. This prior art construction made it necessary to provide a recording switch between one extremity of the fixed winding and the ground, and a reproducing switch between the other extremity of the fixed winding and the ground. Both recording and reproducing switches took the form of semiconductor switches such as transistors. The recording switch was closed for recording, and the reproducing switch was closed for reproduction.
This prior art construction is a source of a problem because the semiconductor switches could not possibly be held completely open. That was because the semiconductor switches when open had some stray capacitances that provided electrical connections between the signal lines and the ground. Moreover, such capacitances tended to resonate with the transformer inductance, with a consequent decrease in the frequency range of the recording or reproducing circuit. Furthermore, the high frequency components of the signals being recorded or reproduced were easy to leak through the stray capacitances. An additional objection concerns noise production by the semiconductor switches. Such noise, amplified by the amplifier in the reproducing circuit, deteriorated the signal to noise ratio of the prior art system.
I know another conventional approach to the problem of how to connect the transducers on the rotary drum to the recording and the reproducing circuit. It employed a second, "fixed" transformer, in addition to the first, "rotary" transformer, which was fixedly mounted on a circuit board. One of the two windings of the fixed transformer was connected to the fixed winding of the rotary transformer, and the other winding of the fixed transformer was connected to the recording and the reproducing circuit. Also, as in the first recited conventional scheme, a recording switch and a reproducing switch were connected between the opposite extremities of the second mentioned winding of the fixed transformer and the ground.
This second known approach is also objectionable because the signal lines between the rotary and the fixed transformer were very easy to pick up noise, again resulting in the deterioration of the signal to noise ratio. Another reason is the use of the additional transformer itself, which required a large installation space and added to the manufacturing cost of the apparatus.